Ligand-dependent interaction of nuclear receptors with potential transcriptional intermediary factors (mediators)

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Abstract
The activity of the ligand-inducible activation function 2 (AF-2) contained in the ligand binding domain (LBD) of nuclear receptors (NRs) is thought to be mediated by transcriptional intermediary factors (TIFs). We have recently reported the isolation and characterization of two novel mouse proteins, designated TIF1 and m SUG1, that interact in a ligand-dependent fashion with the LBD (region E) of several NRs in vivo as well as in vitro. Remarkably, these interactions require the conserved core motif of the AF-2 activating domain (AF-2 AD) and can be blocked by AF-2 antagonists. TIF1 and m SUG1 might therefore represent TIFs/mediators for the ligand-dependent AF-2 of NRs. By comparing the interaction properties of these two putative TIFs with different NRs including the oestrogen (ER), thyroid hormone (TR), vitamin D3 (VDR), retinoic acid (RARα) and retinoid X (RXR) receptors, we demonstrate that: (i) RXRα efficiently interacts with TIF1, but not with mSUG1, whereas TRα interacts much more efficiently with mSUG1 than with TIF1, and RARα, VDR and ER efficiently interact with both TIF1 and m S U G l; (ii) the amphipathic a helix core of AF-2 AD is differentially involved in the interactions of RARα with TIF1 and mSUG1l; and (iii) the AF-2 AD cores of RARα and ER are similarly involved in their interaction with TIF1, but not with mSUG1. Thus the interaction interfaces between the various NRs and either TIF1 or mSUG1 may vary depending on the nature of both the receptor and the putative mediator of its AF-2 function. We discuss the possible roles of TIF1 and mSUGl as mediators of the transcriptional activity of the AF-2 of NRs.